Tagging for the renderer
- Don't deliberately enter data incorrectly for the renderer
but we need to live with the phrase that gets used most often. The basic good practice principle is that you avoid using incorrect tags, or otherwise skewing the data you enter, to make things show up in a specific way on the map rendering.
For example, if landuse=industrial shows up as a pink area, and you have a flowerbed full of pink roses, then tagging your flowerbed as landuse=industrial would be incorrect and must be avoided. Instead, you should accurately tag the flowerbed with the type of plants, and improve the renderers instead so that they understand how to show it.
Another example is when someone abbreviates names, or puts spaces into them like T o w n H a l l S t r e e t to ensure that the names show up "nice" on the map. This should be avoided as it is tagging for the renderer. It breaks the search, and, in addition, map prettiness is subjective. It should be the renderers' job to abbreviate street names or interspace characters.
Proving that every rule needs breaking occasionally, Blackadder famously threw all standards out of the window to make a map of CERN by tagging the particle accelerator rings as highway=trunk and highway=primary (with tunnel=yes) even though they aren't major roads of any kind - he simply liked the colours and knew that they would show up. Don't follow his example! (Update: it has since been retagged to highway=path/tunnel=yes/access=restricted.)
Also, although the most common form of data skewing would be misuse of tags, this rule can apply to the manner in which elements are geometrically arranged.
A misunderstanding comes when people say that you shouldn't tag something "for the renderer" even though the tags being used are accurate and not misleading. For example, if a specialist map renders a particular specialist tag (e.g. the nesting site of rare birds) then using the tags the renderer understands is a perfectly reasonable thing to do, even if they haven't been formally "approved".